Provenance of Bengal Shelf Sediments: 2. Petrology and Geochemistry of Sand

Abstract

The Bangladesh lowlands are traversed by the largest sediment flux on the planet. Detritus generated mostly in Himalayan highlands and conveyed through the Ganga–Brahmaputra rivers and Meghna estuary reaches the Bay of Bengal, where it forms a composite deltaic system. This study integrates the vast existing database on Ganga–Brahmaputra sediments of all grain sizes from clay to sand with new petrographic, mineralogical, and geochemical data on estuarine and shallow-marine sands. A large spectrum of compositional signatures was used to: (i) assess the relative supply of the Ganga and Brahmaputra rivers to estuarine and shelfal sediments; (ii) define the compositional variability of estuarine sediments and the impact exerted by hydraulic sorting and climate-related chemical weathering on provenance signals; (iii) define the compositional variability of shelf sediments and the potential hydrodynamic segregation of fast-settling heavy minerals in coastal environments and of slow-settling platy micas on low-energy outer-shelf floors; (iv) consider the potential additional mud supply from the western subaerial part of the delta formerly built by the Ganga River; and (v) draw a preliminary mineralogical comparison between fluvio-deltaic sediments and turbidites of the Bengal–Nicobar deep-sea fan, thus tracing sediment dispersal across the huge sedimentary system extending from Tibet to the equatorial Indian Ocean. All investigated mineralogical and geochemical parameters, as well as Sr and Nd isotope ratios and clay–mineral assemblages, showed a clear prevalence in sediment supply from the Brahmaputra (60–70%) over the Ganga (30–40%). Heavy-mineral suites and Sr and Nd isotope fingerprints of Bengal shelf sediments are nearly identical to those of the Brahmaputra River and Meghna estuary, also because the Brahmaputra carries almost twice as many Ca-plagioclase grains and heavy minerals including epidote than the Ganga, and these minerals control the large majority of the Sr and Nd budgets. The experience gained in modern settings can be directly extrapolated only to the recent past, because sediments older than the late Pleistocene and buried more than a few hundred meters begin to lose less durable ferromagnesian minerals by selective chemical dissolution, which makes quantitative estimates progressively less robust in more deeply buried older strata.

Provenance of Bengal Shelf Sediments: 1. Mineralogy and Geochemistry of Silt

“…The chambers of foraminiferal tests are commonly filled by glaucony, testifying to very slow accumulation rate [94]. This faunal assemblage indicates original deposition on a shallow-water inner shelf, followed by reworking, winnowing, and sediment starvation during the Holocene transgression [8]. The general lack of Indian Ocean sites in which ooids are forming at present [95,96] confirms the relict character of these palimpsest outer-shelf deposits [97].…”

“…Mostly benthic foraminifera and fragments of mollusks and echinoids occur, associated with and commonly at the core of ooids, in very-fine-grained outer-shelf sand (sample SO347 in Figure 1; present water depth 126 m b.s.l. ; Figure 2I in Garzanti et al [8]). Foraminifera are well preserved and include miliolids (e.g., Triloculina, Quinqueloculina, Peneroplis), rotaliids (e.g., Cibicides), textularids (e.g., Textularia), bolivinids, and rare globigerinids.…”

“…The thorough investigation of the mineralogical variability associated with hydrodynamic sorting and other physical and chemical processes in shallow-marine environments represents the fundamental pre-requisite to link the orogenic provenance signatures of Ganga-Brahmaputra sediments with the deep-sea sedimentary [7]. White circles indicate samples studied in the companion paper [8].…”

“…The thorough investigation of the mineralogical variability associated with hydrodynamic sorting and other physical and chemical processes in shallow-marine environments represents the fundamental pre-requisite to link the orogenic provenance signatures of Ganga-Brahmaputra sediments with the deep-sea sedimentary record [13][14][15][16]. This article and the companion paper dedicated to Bengal shelf sand [8] are, thus, intended as complementary to previous studies carried out on both bedload and suspended-load sediments of the Ganga and Brahmaputra rivers in the proximal part of the sediment-routing system [17][18][19][20][21][22][23][24][25][26][27].…”

Gravimetric Separation of Heavy Minerals in Sediments and Rocks

“…The amount of sample needed for heavy-mineral separation depends on several factors, including expected heavy-mineral concentration, grain size, sorting, and type of study. Heavy-mineral concentration ranges widely from typically << 1% in case of ancient sandstone or quartzose modern sand heavily weathered in equatorial environments or recycled from ancient quartzarenites (e.g., [22]) to 5%-10% in modern orogenic sediments, reaching up to >50% in placer sands (e.g., [23]).…”